Electrical energy storage device utilizing an electrode composed of an amorphous carbon and sulfur-carbon complex

ABSTRACT

The energy storage capacity of a carbon electrode can be increased by advantageously incorporating additive elements into the electrode by the process of initially heat-treating the carbon electrode at a temperature between 700* and 1,000* C under a partial vacuum; subsequently exposing the electrode to the vapors of the desired compound under a slightly positive pressure and at a temperature below about 1,000* C; followed by cycling the electrode alternately in a charge and discharge direction in a cell containing a fused salt electrolyte composed of the halides of alkali metals or alkaline earth metals or their mixtures.

United States Patent 1191 Metcalfe, III et al.

1111 3,811,947 1451 May 21,1974

[54] ELECTRICAL ENERGY STORAGE DEVICE 3,476,603 11/1969 Rafos 136 22UTILIZING AN ELECTRODE COMPOSED 3,639,174 2/1972 Kegelman l36/6 LN OF ANAMORPHOUS CARBON AND 3,645,792 Z/l972 Hacha l36/83 R X SULFUR-CARBONCOMPLEX [75] Inventors: Joseph E. Metcalfe, ITI, Bedford primaryExaminer Amhony skapars Helghts; Robert Rlghtmlle, Attorney, Agent, orFirm-John F. Jones; Sherman J. Northfield; Allan Malse, Kemmer; EvelynR. Kosman Independence, all of Ohio [73] Assignee: The Standard OilCompany,

Cleveland, Ohio 22 Filed: Jan. 22, 1973 [57] ABSTRACT [21] Appl' 325389The energy storage capacity of a carbon electrode can Related US.Application Data be increased by advantageously incorporating additive[63] Continuation of Ser. No. 126,807 March 22, 1971 elements into theelectrode by the process of initially P No, 3,762,954 heat-treating thecarbon electrode at a temperature between 700 and 1,000 C under apartial vacuum; 52 us. c1. 136/6 LF, 136/22, 136/100 R subsequentlyexposing the electrode to the vapors 0f 51] 1m. (:1. H0lm 35/02 thedesired compound under a Slightly Positive p [58] Field of Search 136/6LN, 6 LF, 22,20, Sure and at a'temperature below about l- 13 /33 R,121-122 137 155 100 lowed by cycling the electrode alternately in acharge 252 439 502 and discharge direction in a cell containing a fusedsalt electrolyte composed of the halides of alkali met- [56] ReferencesCi d als or alkaline earth metals or their mixtures.

UNITED STATES PATENTS 3,447,968 6/I969 Rightmire 136/6 LF 6 Claims, 1Drawing Figure I 2 I T6Cl VAPOR ABSORBED 4 a i g; 2.0 I

:l WCI5 VAPOR 5 UNTREATED zssomazo g CARBON 2 5 o 1.0 k g SULFUR VAPORABSORBED CARBON CAPACITY (AMPERE/HRS/IN CELL VOLTAGE (CARBON CATHODE vs.LITHIUM-ALUMINUM ANODE) PATENTED M21 I974 TeCl VAPOR ABSORBED UNTREATEDCARBON wa VAPOR ABSORBED \SULFUR VAPOR ABSORBED CARBON CAPACITY.(AMPERE/HRS/IN ELECTRICAL ENERGY STORAGE DEVICE UTILIZING AN ELECTRODECOMPOSED OF AN AMORPHOUS CARBON AND SULFUR-CARBON COMPLEX This is acontinuation application of our co-pending patent application Ser. No.126,807 filed Mar. 22, 1971, now U.S. Pat. No. 3,762,954.

This invention relates to a process for increasing the energy storagecapacity of an electrical energy storage device. More particularly thisinvention relates to a process for increasing the electrical energystorage capacity of a carbon electrode employed in said electricalenergy storage device by advantageously incorporating additive elementsinto the electrode by means of vapor absorption.

Although this process may be employed as a means for the addition of anyadditive element or compound that will increase the capacity of a carbonelectrode, that can be volatilized under the conditions of the process,and that can be readily absorbed by the carbon, this processparticularly pertains to additives containing the elements of tellurium,tungsten or sulfur. The electrical energy storage capacity of a carbonelectrode may be appreciably enhanced and in some instances virtuallydoubled by vapor absorption of certain compounds of the elements oftellurium, tungsten or sulfur, and their mixtures, in the manner hereindescribed.

The additive-containing carbon electrode produced by the process of thisinvention functions as a reversible positive electrode in an electricalenergy storage system wherein the negative electrode comprises aluminumor an alloy of aluminum, and the electrolyte is a fused salt composed ofthe halides of alkali metals or alkaline earth metals, or mixturesthereof.

In accordance with this invention, improved electrode performance isreadily obtained by incorporating a suitable compound of the desiredelement into the carbon electrode by the process of initiallyheattreating the carbon electrode at a temperature ofup to l,000 C undera partial vacuum to remove oxygen, hydrogen and water therefrom;subsequently exposing the electrode to the vapors of the desiredcompound under a slightly positive pressure and at a temperature abovethe boiling point of the compound and below about l,000 C; followed bycycling the electrode alternately in a charge and discharge direction ina cell containing a fused salt electrolyte composed of the halides ofalkali metals or alkaline earth metals and their mixtures.

Although it is possible to incorporate additives into thecarbon-electrode by other methods such as physically mixing the additiveelement or compound with the carbon, the vapor absorption techniquedescribed in this invention is the preferred method.

The vapor absorption technique has a number of outstanding advantagesover other methods most often used for similar purposes. The compositionand the performance of the electrodes treated in accordance with thepresent invention are readily reproduced. A cell containing an electrodeproduced by this process has a characteristically low leakage currentand high utilization efficiency of the additive element. The compositionof the electrode is very stable, and continuous over-charge can besustained over a long period of time without observing any loss of theadditive element or decline in capacity.

In a preferred mode of preparing the electrode of this invention thecarbon electrode is heated to a temperature of from about 700 to l,000 Cunder a pressure of less than one atmosphere for a period of about 2 to10 hours; followed by exposing the electrode to a flow of vapors of theelement or a desired compound of the elementsof tellurium, tungsten orsulfur at a slightly positive pressure and at a temperature at whichsufficient vapor pressure of the element or compound is generated up toabout l,000 C, for a period of from 2 to 10 hours; and subsequentlyimmersing the additivecontaining electrode in a cell containing anegative electrode consisting essentially of a lithium-aluminum alloyand a molten salt electrolyte composed of a mixture of potassiumchloride and lithium chloride or potassium bromide and lithium bromide,and cycling the cell between the limits of about 1.0 to 3.3 volts.Cycling causes the formation of an electrochemically active species ofthe carbon with the additive element.

The nature of the active species or complex thus formed is notdefinitely known. It is postulated that on absorption, the additiveelement forms a surface complex with the carbon, then on alternatecharge and discharge of the cell, the bond is strengthened and theadditive element becomes permanently bonded to the carbon. In instanceswhere the electrode is composed of graphite, it is possible for someelements to form intercalation compounds with the graphite. Althoughsome additional capacity can be derived from such compounds, a muchgreater increasein capacity is derived from the complex'formed with theadditive element and the amorphous, porous carbon employed in thisinvention. The bond between the amorphous carbon and the additiveelement tellurium, tungsten or sulfur is manifested by a characteristichigher average discharge voltage occurring in the discharge profile ofthe cell.

The additives may be added to the system in the form of any compoundthat is readily vaporized under l ,000 C, and iscompatible with the ionsof the system so that elements foreign to the system will neithercontaminate nor plate out on the surfaces of the electrodes. Examples ofcompounds that are suitable for this purpose include the halides oftellurium, tungsten and sulfur, tungsten oxychlorides, sulfuroxychlorides, elemental sulfur, and the like. The preferred compoundsare those containing anions that are already present in the cell system.Those particularly suitable are the halides of tellurium, tungsten andsulfur, and elemental sulfur.

The concentration of tellurium, tungsten or sulfur required in thecarbon electrode to bring about a discernible enhancement in energystorage capacity is in the range of'about 5 to 40 percent by weight ofthe additive element based on the weight of the carbon and preferablyshould consist of amounts of from about 5 to 35 percent by weight, basedon the weight of carbon.

Since absorptivity is related to carbon source, the carbon comprisingthe cathode in this invention is one that will readily absorb theadditive compounds. Preferably the carbon is an amorphous, highlyporous, high surface area carbon in the from of finely dividedparticulate material. A broad range of carbons is suitable for thispurpose. Carbons in accordance with the present invention can bederived-from activated petroleum coke, wood char, activated sodiumlignosulfonate char, activated bituminous coal, polyvinylidene chloridechars, polyacrylonitrile chars and the like. The active carbon has asurface area in the range of 1002,000 m lg, and most preferredly in therange of 300-1,500 m /g, as measured by the Brunauer-Emmett-Tellermethod. The surface area of the carbon is mainly internal and may begenerated by activation. The pores in the activated carbon must be ofsufficient size to permit additive and electrolyte penetration. Thecarbon composition need not be limited to these types of carbon,however. A very useful polymeric electrode material can be obtained bypolymerizing a mixture of a vinyl nitrile monomer and a polyalkenylmonomer containing at least two polymerizable alkenyl groups, as morefully described in US. Pat. No. 3,476,603, and carbonizing same.

The additive-containing carbon electrode of this invention isparticularly adapted to be used in an electrical energy storage cellwhere the negative electrode may comprise any one of several differentmetals or metal alloys that are stable in the electrolyte melt'of alkalimetal or alkaline earth metal halides. For example, the negativeelectrode may be composed of a metal such as lithium, sodium, potassium,magnesium, bismuth, or antimony, or alloys of these metals. Lithium isparticularly suitable and alloysof lithium with such metals as aluminum,indium, tin, lead, silver and cop per may also be employed. Ternarylithium alloys can likewise be used. Especially preferred is analuminumlithium electrode which can be produced by preparing an alloy ofaluminum and lithium, or, alternatively, by preconditioning or cycling asubstantially pure aluminum electrode in an electrolyte containinglithium ions, during which preconditioning process lithium is diffusedinto the aluminum electrode structure. The former is the preferredembodiment.

'The preferred aluminum-lithium alloy of the electrode comprisesaluminum in amounts of from about 70 to 95 weight percent and from about5 to 30 weight percent of lithium, based on total composition.Incidental impurities such as, for example, copper, magnesium,manganese, indium and iron may be present in quantities less than weightpercent, based on total composition. An aluminum-lithium electrode ofthis range of composition operates at substantially constant voltage andexhibits high electrical energy storagecapabilities.

The electrolyte used in the device of this invention is a fused saltmixture containing alkali metal and alkaline earth metal halides, as forexample lithium chloride, potassium chloride, sodium chloride, calciumchloride, calcium fluoride, magnesium chloride, lithium bromide andpotassium bromide. The lowest melting point media are most desirable.However, it is contemplated by the present invention that the medium beoperable in the liquid state at temperatures in the range of 350 to 600C.

Typical examples of materials which can be used as binary saltelectrolytes include lithium chloridepotassium chloride, potassiumchloride-magnesium chloride, magnesium chloride-sodium chloride, lithiumbromide-potassium bromide, lithium fluoride-rubidium fluoride, magnesiumchloride-rubidium chloride, lithium chloride-lithium fluoride, lithiumchloridestrontium chloride, cesium chloride-sodium chloride, calciumchloride-lithium chloride, and mixtures thereof.

Examples of ternary electrolytes are calcium chloride lithium chloridepotassium chloride, lithium chloride-potassium chloride-sodium chloride,lithium chloride-potassium chloride-magnesiumchloride, calciumchloride-lithium chloride-sodium chloride, and lithium bromide-sodiumbromide-lithium chloride.

The preferred electrolyte systems are those of potassiumchloride-lithium chloride and lithium bromide and potassium bromide, andmixtures thereof. A lithium chloride-potassium chloride system of 41mole percent potassium chloride-and 59 mole percent lithium chlorideforms a eutectic which melts at 352 C and has a decomposition voltage ofabout 3.3 volts.

To insure good electrical structure and full capacity of the electricalenergy storage cell, easily degradable components in the structure areremoved and the electrodes are permeated with electrolyte for maximumoperational efficiency. This is accomplished by preconditioning theelectrical energy storage cell by immersing the positivecarbon-containing electrode and the negative electrode in theelectrolyte and alternately charging and discharging the cell at aconstant predetermined voltage. This cycling converts the carbon into agood electron conductor or negative charge holding medium, and causesthe electrochemical association of the carbon with certain constituentsfrom the eutectic melt of the electrolyte.

The formation of the electrochemically produced active tellurium,tungsten or sulfur species in the carbonv electrode may take placeconcurrently with the preconditioning treatment of the electrode, bycycling the cell between the voltage limits of about 1.0 to 3.3 volts.

This invention will be further illustrated by reference to the followingexamples. The examples are illustrations of specific embodiments of theinvention and are not to be construed in any way as limitations of theinvention.

The experiments were carried out in a stainless steel 7 test tube cell.The carbon electrode was fixed rigidly to a graphite current carrier andthe negative metallic electrode was fixed rigidly to a steel currentcarrier. The container comprising the electrolyte and electrodes waspurged of atmospheric air and an inert gas was introduced into thecontainer and then sealed.

EXAMPLE 1 A carbon cathode having the dimensions of 1%" X 1%" X 0.025was prepared from a commercial grade carbon (No.4052) produced by PureCarbon Company, and having the following physical properties:

Surface area 450 m /g Density 0.81 g/cc Porosity 47-51% (vol.)

The electrode was vacuum heat-treated at about l,000 C for a period of 8hours, at a pressure of less than 1 atmosphere. It was thenexposedtovapors of tellurium tetrachloride (TeCl at 454 C and latmosphere of pressure for 4 hours. After exposure to the telluriumtetrachloride vapors in a sealed steel container, the electrode showed aweight gain of 8 glini, which included TeCl, absorbed on the graphiteheader. The electrode was then placed in a stainless steel cellassembly, as described above, containing an aluminum- ]ithium alloyanode having the dimensions 3" X 2" X 0.03 and initially containing 1 1percent by weight of lithium, and a molten salt electrolyte composed ofa eutectic mixture of 41 mole percent potassium chloride and 59 molepercent lithium chloride. An argon atmosphere was established in thecell and the cell was operated at a temperature of 450 C. The cellcycled for approximately 50 cycles between the voltage limits of 3.28and 1 volt across the cell, and discharged at a constant current of 500milliamperes/in. of cathode. Under these conditions, thetellurium-containing carbon electrode had a capacity of 4.03 amperehours/in. of cathode.

EXAMPLE 2 An electrode having the same carbon composition and dimensionsas that of Example 1 was vacuum heattreated at l,000 C at less than 1atmosphere of pressure for 7 hours and was then exposed to vapors oftungsten hexachloride (WCl at 460 C at 1 atmosphere of pressure for 2hours. After exposure to the WCl vapors, the electrode showed a weightgain of 2.5 g/in. of carbon (including WCl absorbed on the graphiteheader). The electrode was then placed in a cell assembly as describedin Example 1 and was cycled between 1.0 volt and 3.28 volts at aconstant current discharge of 500 milliamperes/in. of cathode. Thecapacity of the electrode was 3.86 ampere hours/in. of carbon.

EXAMPLE 3 The tungsten-containing carbon electrode of Example 2 wascycled for 41 cycles in the same cell assembly as in Example 2, between1.0 volt and 3.28 volts open circuit. On the 41st cycle, the electrodewas overcharged (chlorine evolved at the carbon electrode) at a constantcurrent of approximately 500 milliamperes/in. of the carbon electrodefor 1 hour. The discharge of the electrode following this charge showedno loss in capacity.

EXAMPLE 4 A carbon electrode was prepared having the composition 60weight percent of a porous activated carbon having a density of0.76-0.79 g/cc, a surface area of 800 m /g and a U.S. standard mesh sizeof 100-140 (from Pittsburgh Activated Carbon Co.), weight percentgraphite and 20 weight percent of a binder comprising a thermosettingphenolic-based resin (Borden SD. 5143). The components were mixed in thedesired proportions and compressed into a carbon plate having thedimensions 1%" X 1%" X 0.025". The electrode was heat-treated andsubsequently exposed to vapors of tungsten hexachloride (WCI under thesame conditions as described in Example 2 and a weight gain of 13.1g/in. of cathode was obtained (including WCl deposited on graphiteheader.) On cycling the electrode in a cell as in Example 1, a totalelectrode capcity of approximately 3.4 ampere hours/in. of cathode wasobtained at a constant discharge current of 500 milliamperes/in.cathode.

EXAMPLE 5 A carbon electrode of the carbon composition and dimensions ofExample 1 was vacuum heat-treated at 998 C under a pressure of less than1 atmosphere for 6 2 hours. The electrode was then exposed to sulfurvapors at 475 C at a pressure of slightly above 1 atmosphere for 3hours. After exposure to sulfur vapors the electrode showed a weightgain of 10.30 g/in. of cathode. On cyclng the electrode in a cell as inthe above examples at a constant current discharge of 500milliamperes/in. of cathode, the sulfur-containing carbon electrode hada capacity of 2.76 ampere hours/in." of cathode.

EXAMPLE 6 An untreated carbon electrode having the same carboncomposition and dimensions as indicated in Example l was placed in acell and was cycled in the same manner as described in this example. Theuntreated carbon electrode had a measured capacity of 1.96 amperehours/in. of carbon electrode.

The discharge cell voltage curves for the additivecontaining carbonelectrodes of Examples 1, 2 and 5 compared with the untreated electrodeof the same carbon composition of Example 6 are shown in theaccompanying FIGURE. The data illustrate the improvement obtained in thecapacities as well as the plateaus appearing in the discharge curves forthe carbon electrodes containing the tellurium, tungsten and sulfuradditives, and the gradual downward slope of the discharge curve for theuntreated carbon electrode. The effect on capacity of vapor absorptionof tungsten halide on an electrode composed of a mixture of graphite andamorphous, activated carbon is illustrated by Example 4. The data showthat the tungsten additive is considerably less effective in increasingthe capacity of an electrode containing graphite.

EXAMPLE 7 An electrode of the carbon composition and dimensions of thatin Example 1 was heat-treated at 250 C for 3 hours under a pressure ofless than 1 atmosphere. The electrode was then exposed'to sulfur vaporsat 470 C and 1 atmosphere of pressure for 3 hours. On opening thereaction vessel, significant amounts of byproducts were found on thesurface of the carbon and on the container walls. When placed in a cellas in Example 5, and cycled in the same-manner, the electrode did nothave a well-defined plateau and showed a lower average voltage ondischarge as compared with that of the sulfur-containing electrode thathad been heattreated at approximately l,000 C. The capacity of theelectrode in this experiment was 2.18 ampere hours/in. of carbon.

We claim:

I. An electrical energy storage device comprising:

a. a container;

b. an electrolyte in said container composed of a salt selected from thegroup consisting of a halide of an alkali metal or an alkaline earthmetal or mixtures thereof;

c. a negative electrode in contact with said electrolyte; and

d. a positive electrode composed of an amorphous carbon and anelectrochemically-formed sulfurcarbon complex also in contact with saidelectrolyte;

said device being operable at a temperature at which the electrolyte isin a fused state.

2. The electrical energy storage device of claim 1 wherein the positiveelectrode (a) is composed of an activated,.amorphous carbon and anelectrochemically-formed sulfur-carbon complex.

lide-containing salt.

6. The electrochemical energy storage device of claim 5 wherein theelectrolyte consists essentially of a eutectic mixture of lithiumchloride and potassium chloride and the negative electrode consists ofan alloy having the composition to weight percent aluminum and 5 to 30weight percent lithium.

2. The electrical energy storage device of claim 1 wherein the positiveelectrode (a) is composed of an activated, amorphous carbon and anelectrochemically-formed sulfur-carbon complex.
 3. The electrical energystorage device of claim 2 where the positive electrode contains fromabout 5 to 40 percent by weight of sulfur, based on the weight of thecarbon.
 4. The electrical energy storage device of claim 3 wherein thenegative electrode in (c) consists essentially of an alloy of lithiumand aluminum.
 5. The electrochemical energy storage device of claim 4wherein the electrolyte comprises a lithium halide-containing salt. 6.The electrochemical energy storage device of claim 5 wherein theelectrolyte consists essentially of a eutectic mixture of lithiumchloride and potassium chloride and the negative electrode consists ofan alloy having the composition 70 to 95 weight percent aluminum and 5to 30 weight percent lithium.